A method has already been known to heat-curing a cyanic acid ester and/or its prepolymer using a catalyst to obtain a cured resin. This resin exhibits superior mechanical and electrical properties at room temperature and elevated temperatures, and can find various applications. For example, it is impregnated as a binder in a base material made of cellulosic fibers or glass fibers, and the impregnated base material is used as a laminated plate, or processed into press-formed articles. Moreover, this resin can be used as an insulating varnish for coils, or as molding materials.
In the conventional polymerization of cyanic acid esters and/or their prepolymers to form cured products, various acids, bases, salts and phosphorus compounds are used in combination as catalysts. A combination of a metal salt of an organic acid such as zinc octoate, catechol and triethylene diamine has been in frequent use as a suitable catalyst. However, triethylene diamine used as one ingredient of this catalyst has acute toxicity such as the irritation to the skin. Coupled with its sublimable nature, the toxicity gives rise to the disadvantage that triethylene diamine must be handled with care. Its strong hygroscopicity also presents handling difficulties. Moreover, a varnish prepared by blending the above catalyst with a cyanic acid ester and/or its prepolymer has poor storage stability. In other words, under pre-designed use conditions, triethylene diamine shortens the pot life of the varnish. Furthermore, when triethylene diamine is used, the rate of polymerization of the cyanic acid ester or its prepolymer is greatly affected by the temperature, and drastically changes according to even a slight change in temperature. It is extremely difficult therefore to adjust the B-stage of the curing of the varnish to the desired degree. The properties of the cured resin are neither satisfactory, for example, by showing a degradation in properties under a hygroscopic condition.